The transforming src genes of Harvey (Ha-) and Kirsten murine sarcoma virus (Ki-MuSV) encode p21. While these viruses encode highly related p21 polypeptides, their src genes are derived from cellular sarc genes which are distinct in all species studied. We have cloned two rat cellular sarc genes related to Ha-MuSV src. One gene is colinear with viral src while the other has three introns in its coding sequence. Nevertheless, both genes are capable of transforming fibroblasts and inducing high levels of p21 expression following DNA transfection. This project has involved recombinant DNA technology and other biochemical analyses.